Information About Digital Certificates

Digital certificates provide digital identification for authentication. A digital certificate includes information that identifies a device or user, such as the name, serial number, company, department, or IP address. CAs are trusted authorities that "sign" certificates to verify their authenticity, thereby guaranteeing the identity of the device or user. CAs issue digital certificates in the context of a PKI, which uses public-key or private-key encryption to ensure security.

For authentication using digital certificates, at least one identity certificate and its issuing CA certificate must exist on an adaptive security appliance. This configuration allows multiple identities, roots, and certificate hierarchies. Descriptions of several different types of available digital certificates follow:

•A CA certificate is used to sign other certificates. It is self-signed and called a root certificate. A certificate that is issued by another CA certificate is called a subordinate certificate. For more information, see the "Configuring CA Certificate Authentication" section.

•Code-signer certificates are special certificates that are used to create digital signatures to sign code, with the signed code itself revealing the certificate origin. For more information, see the "Configuring Code Signer Certificates" section.

Note CA certificates and identity certificates apply to both site-to-site VPN connections and remote access VPN connections. Procedures in this document refer to remote access VPN use in the ASDM GUI.

CAs are responsible for managing certificate requests and issuing digital certificates. A digital certificate includes information that identifies a user or device, such as a name, serial number, company, department, or IP address. A digital certificate also includes a copy of the public key for the user or device. A CA can be a trusted third party, such as VeriSign, or a private (in-house) CA that you establish within your organization.

Public Key Cryptography

Digital signatures, enabled by public key cryptography, provide a way to authenticate devices and users. In public key cryptography, such as the RSA encryption system, each user has a key pair containing both a public and a private key. The keys act as complements, and anything encrypted with one of the keys can be decrypted with the other.

In simple terms, a signature is formed when data is encrypted with a private key. The signature is attached to the data and sent to the receiver. The receiver applies the public key of the sender to the data. If the signature sent with the data matches the result of applying the public key to the data, the validity of the message is established.

This process relies on the receiver having a copy of the public key of the sender and a high degree of certainty that this key belongs to the sender, not to someone pretending to be the sender.

Obtaining the public key of a sender is normally handled externally or through an operation performed at installation. For example, most web browsers are configured with the root certificates of several CAs by default. For VPN, the IKE protocol, a component of IPSec, can use digital signatures to authenticate peer devices before setting up security associations.

Certificate Scalability

Without digital certificates, you must manually configure each IPSec peer for each peer with which it communicates; as a result, each new peer that you add to a network would require a configuration change on each peer with which it needs to communicate securely.

When you use digital certificates, each peer is enrolled with a CA. When two peers try to communicate, they exchange certificates and digitally sign data to authenticate each other. When a new peer is added to the network, you enroll that peer with a CA and none of the other peers need modification. When the new peer attempts an IPSec connection, certificates are automatically exchanged and the peer can be authenticated.

With a CA, a peer authenticates itself to the remote peer by sending a certificate to the remote peer and performing some public key cryptography. Each peer sends its unique certificate, which was issued by the CA. This process works because each certificate encapsulates the public key for the associated peer, each certificate is authenticated by the CA, and all participating peers recognize the CA as an authenticating authority. The process is called IKE with an RSA signature.

The peer can continue sending its certificate for multiple IPSec sessions, and to multiple IPSec peers, until the certificate expires. When its certificate expires, the peer administrator must obtain a new one from the CA.

CAs can also revoke certificates for peers that no longer participate in IPSec. Revoked certificates are not recognized as valid by other peers. Revoked certificates are listed in a CRL, which each peer may check before accepting a certificate from another peer.

Some CAs have an RA as part of their implementation. An RA is a server that acts as a proxy for the CA, so that CA functions can continue when the CA is unavailable.

Key Pairs

Key pairs are RSA keys, which have the following characteristics:

•RSA keys can be used for SSH or SSL.

•SCEP enrollment supports the certification of RSA keys.

•For the purposes of generating keys, the maximum key modulus for RSA keys is 2048 bits. The default size is 1024. Many SSL connections using identity certificates with RSA key pairs that exceed 1024 bits can cause a high CPU usage on the adaptive security appliance and rejected clientless logins.

•You can generate a general purpose RSA key pair, used for both signing and encryption, or you can generate separate RSA key pairs for each purpose. Separate signing and encryption keys help to reduce exposure of the keys, because SSL uses a key for encryption but not signing. However, IKE uses a key for signing but not encryption. By using separate keys for each, exposure of the keys is minimized.

Trustpoints

Trustpoints let you manage and track CAs and certificates. A trustpoint is a representation of a CA or identity pair. A trustpoint includes the identity of the CA, CA-specific configuration parameters, and an association with one, enrolled identity certificate.

After you have defined a trustpoint, you can reference it by name in commands requiring that you specify a CA. You can configure many trustpoints.

Note If an adaptive security appliance has multiple trustpoints that share the same CA, only one of these trustpoints sharing the CA can be used to validate user certificates. To control which trustpoint sharing a CA is used for validation of user certificates issued by that CA, use the support-user-cert-validation command.

For automatic enrollment, a trustpoint must be configured with an enrollment URL, and the CA that the trustpoint represents must be available on the network and must support SCEP.

You can export and import the keypair and issued certificates associated with a trustpoint in PKCS12 format. This format is useful to manually duplicate a trustpoint configuration on a different adaptive security appliance.

Certificate Enrollment

The adaptive security appliance needs a CA certificate for each trustpoint and one or two certificates for itself, depending upon the configuration of the keys used by the trustpoint. If the trustpoint uses separate RSA keys for signing and encryption, the adaptive security appliance needs two certificates, one for each purpose. In other key configurations, only one certificate is needed.

The adaptive security appliance supports enrollment with SCEP and with manual enrollment, which lets you paste a base-64-encoded certificate directly into the terminal. For site-to-site VPNs, you must enroll each adaptive security appliance. For remote access VPNs, you must enroll each adaptive security appliance and each remote access VPN client.

Revocation Checking

When a certificate is issued, it is valid for a fixed period of time. Sometimes a CA revokes a certificate before this time period expires; for example, because of security concerns or a change of name or association. CAs periodically issue a signed list of revoked certificates. Enabling revocation checking forces the adaptive security appliance to check that the CA has not revoked a certificate each time that it uses the certificate for authentication.

When you enable revocation checking, the adaptive security appliance checks certificate revocation status during the PKI certificate validation process, which can use either CRL checking, or OCSP, or both. OCSP is only used when the first method returns an error (for example, that the server is unavailable).

With CRL checking, the adaptive security appliance retrieves, parses, and caches CRLs, which provide a complete list of revoked certificates. The ASA evaluates certificates against CRLs, also called authority revocation lists, all the way from the identity certificate up the chain of subordinate certificate authorities.

OCSP offers a more scalable method of checking revocation status in that it localizes certificate status through a validation authority, which it queries for status of a specific certificate.

CRLs

CRLs provide the adaptive security appliance with one way of determining whether a certificate that is within its valid time range has been revoked by the issuing CA. CRL configuration is part of configuration of a trustpoint.

You can configure the adaptive security appliance to make CRL checks mandatory when authenticating a certificate by using the revocation-check crl command. You can also make the CRL check optional by using the revocation-check crl none command, which allows the certificate authentication to succeed when the CA is unavailable to provide updated CRL data.

The adaptive security appliance can retrieve CRLs from CAs using HTTP, SCEP, or LDAP. CRLs retrieved for each trustpoint are cached for a configurable amount of time for each trustpoint.

When the adaptive security appliance has cached a CRL for longer than the amount of time it is configured to cache CRLs, the adaptive security appliance considers the CRL too old to be reliable, or "stale." The adaptive security appliance tries to retrieve a newer version of the CRL the next time that a certificate authentication requires a check of the stale CRL.

The adaptive security appliance caches CRLs for an amount of time determined by the following two factors:

•The number of minutes specified with the cache-time command. The default value is 60 minutes.

•The NextUpdate field in the CRLs retrieved, which may be absent from CRLs. You control whether the adaptive security appliance requires and uses the NextUpdate field with the enforcenextupdate command.

The adaptive security appliance uses these two factors in the following ways:

•If the NextUpdate field is not required, the adaptive security appliance marks CRLs as stale after the length of time defined by the cache-time command.

•If the NextUpdate field is required, the adaptive security appliance marks CRLs as stale at the sooner of the two times specified by the cache-time command and the NextUpdate field. For example, if the cache-time command is set to 100 minutes and the NextUpdate field specifies that the next update is 70 minutes away, the adaptive security appliance marks CRLs as stale in 70 minutes.

If the adaptive security appliance has insufficient memory to store all CRLs cached for a given trustpoint, it deletes the least recently used CRL to make room for a newly retrieved CRL.

OCSP

OCSP provides the adaptive security appliance with a way of determining whether a certificate that is within its valid time range has been revoked by the issuing CA. OCSP configuration is part of trustpoint configuration.

OCSP localizes certificate status on a validation authority (an OCSP server, also called the responder) which the adaptive security appliance queries for the status of a specific certificate. This method provides better scalability and more up-to-date revocation status than does CRL checking, and helps organizations with large PKI installations deploy and expand secure networks.

Note The adaptive security appliance allows a five-second time skew for OCSP responses.

You can configure the adaptive security appliance to make OCSP checks mandatory when authenticating a certificate by using the revocation-check ocsp command. You can also make the OCSP check optional by using the revocation-check ocsp none command, which allows the certificate authentication to succeed when the validation authority is unavailable to provide updated OCSP data.

OCSP provides three ways to define the OCSP server URL. The adaptive security appliance uses these servers in the following order:

1. The OCSP URL defined in a match certificate override rule by using the match certificate command).

2. The OCSP URL configured by using the ocsp url command.

3. The AIA field of the client certificate.

Note To configure a trustpoint to validate a self-signed OCSP responder certificate, you import the self-signed responder certificate into its own trustpoint as a trusted CA certificate. Then you configure the match certificate command in the client certificate validating trustpoint to use the trustpoint that includes the self-signed OCSP responder certificate to validate the responder certificate. Use the same procedure for configuring validating responder certificates external to the validation path of the client certificate.

The OCSP server (responder) certificate usually signs the OCSP response. After receiving the response, the adaptive security appliance tries to verify the responder certificate. The CA normally sets the lifetime of the OCSP responder certificate to a relatively short period to minimize the chance of being compromised.The CA usually also includes an ocsp-no-check extension in the responder certificate, which indicates that this certificate does not need revocation status checking. However, if this extension is not present, the adaptive security appliance tries to check revocation status using the same method specified in the trustpoint. If the responder certificate is not verifiable, revocation checks fail. To avoid this possibility, use the revocation-checknone command to configure the responder certificate validating trustpoint, and use the revocation-check ocsp command to configure the client certificate.

•Provides a certificate authority on the adaptive security appliance for use with browser-based and client-based SSL VPN connections.

•Provides trusted digital certificates to users, without the need to rely on external certificate authorization.

•Provides a secure, in-house authority for certificate authentication and offers straightforward user enrollment by means of a website login.

Storage for Local CA Files

The adaptive security appliance accesses and implements user information, issued certificates, and revocation lists using a local CA database. This database resides in local flash memory by default, or can be configured to reside on an external file system that is mounted and accessible to the adaptive security appliance.

No limits exist on the number of users that can be stored in the local CA user database; however, if flash memory storage issues arise, syslogs are generated to alert the administrator to take action, and the local CA could be disabled until the storage issues are resolved. Flash memory can store a database with 3500 users or less; however, a database of more than 3500 users requires external storage.

The Local CA Server

After you configure a local CA server on the adaptive security appliance, users can enroll for a certificate by logging into a website and entering a username and a one-time password that is provided by the local CA administrator to validate their eligibility for enrollment.

As shown in Figure 36-1, the local CA server resides on the adaptive security appliance and handles enrollment requests from website users and CRL inquiries coming from other certificate validating devices and adaptive security appliances. Local CA database and configuration files are maintained either on the adaptive security appliance flash memory (default storage) or on a separate storage device.

Figure 36-1 The Local CA

Licensing Requirements for Digital Certificates

The following table shows the licensing requirements for this feature:

Model

License Requirement

All models

Base License.

Guidelines and Limitations

This section includes the guidelines and limitations for this feature.

Context Mode Guidelines

Supported in single and multiple context mode.

Firewall Mode Guidelines

Supported in routed and transparent mode.

Failover Guidelines

Does not support replicating sessions in Stateful Failover.

IPv6 Guidelines

Supports IPv6.

Additional Guidelines

For adaptive security appliances that are configured as CA servers or clients, limit the validity period of the certificate to less than the recommended end date of 03:14:08 UTC, January 19, 2038. This guideline also applies to imported certificates from third-party vendors.

Configuring CA Certificate Authentication

The CA Certificates pane displays the available certificates, identified by the issued to and issued by CA server, the date that the certificate expires, the associated trustpoints, and the certificate usage or purpose. In the CA Certificates pane, you can perform the following tasks:

•Authenticate self-signed or subordinate CA certificates.

•Install CA certificates on the adaptive security appliance.

•Create a new certificate configuration.

•Edit an existing certificate configuration.

•Obtain a CA certificate manually and import it.

•Have the adaptive security appliance use SCEP to contact the CA, and then automatically obtain and install the certificate.

•Display details and issuer information for a selected certificate.

•Access the CRL for an existing CA certificate.

•Remove the configuration of an existing CA certificate.

•Save the new or modified CA certificate configuration.

•Discard any changes and return the certificate configuration to the original settings.

Adding or Installing a CA Certificate

You can add a new certificate configuration from an existing file, by manually pasting a certificate in PEM format, or by automatic enrollment using SCEP. SCEP is a secure messaging protocol that requires minimal user intervention and lets you enroll and install certificates using only the VPN Concentrator Manager.

Step 6 Copy and paste the PEM format (base64 or hexadecimal) certificate into the area provided, then click Install Certificate.

Step 7 To enroll automatically, click the Use SCEP radio button. The adaptive security appliance contacts the CA using SCEP, obtains the certificates, and installs them on the device. To use SCEP, you must enroll with a CA that supports SCEP, and you must enroll via the Internet. Automatic enrollment using SCEP requires that you provide the following information:

•The path and file name of the certificate to be automatically installed.

•The maximum number of minutes to retry certificate installation.The default is one minute.

•The number of retries for installing a certificate. The default is zero, which indicates unlimited retries within the retry period.

Step 8 To display additional configuration options for new and existing certificates, click More Options.

Note After you delete a certificate configuration, it cannot be restored. To recreate the deleted certificate, click Add to reenter all of the certificate configuration information.

Showing CA Certificate Details

To show detailed information about the selected CA certificate, click Show Details to display the Certificate Details dialog box, which includes the following three display-only tabs:

•The General tab displays the values for type, serial number, status, usage, public key type, CRL distribution point, the times within which the certificate is valid, and associated trustpoints. The values apply to both available and pending status.

•The Issued to tab displays the X.500 fields of the subject DN or certificate owner and their values. The values apply only to available status.

•The Issued by tab displays the X.500 fields of the entity granting the certificate. The values apply only to available status.

Requesting a CRL

To update the current version of the CRL, click Request CRL. CRL updates provide the current status of certificate users. If the request fails, an error message appears. The CRL is updated and regenerated automatically until it expires; clicking Request CRL forces an immediate CRL file update and regeneration.

Configuring CA Certificates for Revocation

To configure CA certificates for revocation, perform the following steps:

Step 3 Check the Use CRL Distribution Point from the certificate check box to direct revocation checking to the CRL distribution point from the certificate being checked.

Step 4 Check the Use Static URLs configured below check box to list specific URLs to be used for CRL retrieval. The URLs you select are implemented in the order in which you add them. If an error occurs with the specified URL, the next URL in order is taken.

Step 5 In the Static Configuration area, click Add.

The Add Static URL dialog box appears.

Step 6 In the URL field, enter the static URL to use for distributing the CRLs, and then click OK.

•To enable LDAP for CRL retrieval, check the Enable Lightweight Directory Access Protocol (LDAP) check box. With LDAP, CRL retrieval starts an LDAP session by connecting to a named LDAP server, accessed by a password. The connection is on TCP port 389 by default. Enter the following required parameters:

Configuring OCSP Rules

The adaptive security appliance examines OCSP rules in priority order, and applies the first one that matches. X.509 digital certificates are an alternative to using CRLs.

Note Make sure that you have configured a certificate map before you try to add OCSP rules. If a certificate map has not been configured, an error message appears. To configure a certificate map, choose Configuration > Site-to-Site VPN > Advanced > Certificate to Connection Profile Maps > Rules > Add.

To configure OCSP rules for obtaining revocation status of an X.509 digital certificate, perform the following steps:

Step 3 Choose the certificate map to match to this OCSP rule. Certificate maps match user permissions to specific fields in a certificate. The name of the CA that the adaptive security appliance uses to validate responder certificates appears in the Certificate field. The priority number for the rule appears in the Index field. The URL of the OCSP server for this certificate appears in the URL field.

Configuring Advanced CRL and OCSP Settings

When a certificate is issued, it is valid for a fixed period of time. Sometimes a CA revokes a certificate before this time period expires; for example, because of security concerns or a change of name or association. CAs periodically issue a signed list of revoked certificates. Enabling revocation checking forces the adaptive security appliance to check that the CA has not revoked the certificate being verified. The adaptive security appliance supports two methods of checking revocation status: CRL and OCSP.

To configure additional CRL and OCSP settings, perform the following steps:

Step 3 In the CRL Options area, enter the number of minutes between cache refreshes. The default is 60 minutes. The range is 1-1440 minutes. To avoid having to retrieve the same CRL from a CA repeatedly, the adaptive security appliance can store retrieved CRLs locally, which is called CRL caching. The CRL cache capacity varies by platform and is cumulative across all contexts. If an attempt to cache a newly retrieved CRL would exceed its storage limits, the adaptive security appliance removes the least recently used CRL until more space becomes available.

Step 4 Check the Enforce next CRL update check box to require valid CRLs to have a Next Update value that has not expired. Uncheck the Enforce next CRL update check box to let valid CRLs with no Next Update value or a Next Update value that has expired.

Step 5 In the OCSP Options area, enter the URL for the OCSP server. The adaptive security appliance uses OCSP servers according to the following order:

1. OCSP URL in a match certificate override rule

2. OCSP URL configured in the selected OCSP Options attribute

3. AIA field of a remote user certificate

Step 6 By default, the Disable nonce extension check box is checked, which cryptographically binds requests with responses to avoid replay attacks. This process works by matching the extension in the request to that in the response, ensuring that they are the same. Uncheck the Disable nonce extension check box if the OCSP server you are using sends pregenerated responses that do not include this matching nonce extension.

Step 7 In the Validation Policy area, choose one of the following options:

•Click the SSL radio button or the IPSec radio button to restrict the type of remote session that this CA can be used to validate.

•Click the SSL and IPSec radio button to let the CA validate both types of sessions.

Step 8 In the Other Options area, choose one of the following options:

•Check the Accept certificates issued by this CA check box to indicate that the adaptive security appliance should accept certificates from the specified CA.

•Check the Accept certificates issued by the subordinate CAs of this CA check box to indicate that the adaptive security appliance should accept certificates from the subordinate CA.

Step 9 Click OK to close this tab, and then click Apply to save your configuration changes.

The Add Identity Certificate dialog box appears, with the selected trustpoint name displayed at the top.

Step 3 To import an identity certificate from an existing file, click the Import the identity certificate from a file radio button.

Step 4 Enter the passphrase used to decrypt the PKCS12 file.

Step 5 Enter the path name of the file, or click Browse to display the Import ID Certificate File dialog box. Find the certificate file, and then click Import ID Certificate File.

Step 6 To add a new identity certificate, click the Add a new identity certificate radio button.

Step 7 Click New to display the Add Key Pair dialog box.

Step 8 To use the default key pair name, click the Use default keypair name radio button.

Step 9 To use a new key pair name, click the Enter a new key pair name radio button, and type the new name. The adaptive security appliance supports multiple key pairs.

Step 10 Choose the modulus size from the drop-down list.

Step 11 Choose the key pair usage by clicking the General purpose radio button (default) or Special radio button. When you choose the Special radio button, the adaptive security appliance generates two key pairs, one for signature use and one for encryption use. This selection indicates that two certificates are required for the corresponding identity.

Step 12 Click Generate Now to create new key pairs, and then click Show to display the Key Pair Details dialog box, which includes the following display-only information:

Showing Identity Certificate Details

To show detailed information about the selected identity certificate, click Show Details to display the Certificate Details dialog box, which includes the following three display-only tabs:

•The General tab displays the values for type, serial number, status, usage, public key type, CRL distribution point, the times within which the certificate is valid, and associated trustpoints. The values apply to both available and pending status.

•The Issued to tab displays the X.500 fields of the subject DN or certificate owner and their values. The values apply only to available status.

•The Issued by tab displays the X.500 fields of the entity granting the certificate. The values apply only to available status.

Deleting an Identity Certificate

To remove an identity certificate configuration, select it, and then click Delete.

Note After you delete a certificate configuration, it cannot be restored. To recreate the deleted certificate, click Add to reenter all of the certificate configuration information.

Exporting an Identity Certificate

You can export a certificate configuration with all associated keys and certificates in PKCS12 format, which is the public key cryptography standard, and can be base64 encoded or in hexadecimal format. A complete configuration includes the entire chain (root CA certificate, identity certificate, key pair) but not enrollment settings (subject name, FQDN and so on). This feature is commonly used in a failover or load-balancing configuration to replicate certificates across a group of adaptive security appliances; for example, remote access clients calling in to a central organization that has several units to service the calls. These units must have equivalent certificate configurations. In this case, an administrator can export a certificate configuration and then import it across the group of adaptive security appliances.

To export an identity certificate, perform the following steps:

Step 1 Click Export to display the Export Certificate dialog box.

Step 2 Enter the name of the PKCS12 format file to use in exporting the certificate configuration. Alternatively, click Browse to display the Export ID Certificate File dialog box to find the file to which you want to export the certificate configuration.

Step 3 Choose the certificate format by clicking the PKCS12 Format radio button or the PEM Format radio button.

Step 4 Enter the passphrase used to encrypt the PKCS12 file for export.

b. Click Show to display the Key Details dialog box, which provides information about the selected key pair, including date and time generated, usage (general or special purpose), modulus size, and key data.

Step 6 Click Generate Request to generate the certificate signing request, which you can then sendto Entrust, or save to a file and send later.

The Enroll with Entrust dialog box appears, with the CSR displayed.

Step 7 To complete the enrollment process, click the request a certificate from Entrust link by copying and pasting the CSR provided and submitting it through the Entrust web form, provided at http://www.entrust.net/cisco/. Alternatively, to enroll at a later time, save the generated CSR to a file, then click the enroll with Entrust link on the Identity Certificates pane to complete the enrollment process.

Step 8 Entrust issues a certificate after verifying the authenticity of your request. which may take several days. You then need to install the certificate by selecting the pending request in the Identity Certificate pane and clicking Install. Click Close to close the Enroll with Entrust dialog box.

Installing Identity Certificates

The Installbutton on the Identity Certificates pane is dimmed unless an enrollment is pending. Whenever the adaptive security appliance receives a CSR, the Identity Certificates pane displays the pending ID certificate. When you select the pending Identity Certificate, the Install button activates.

When you transmit the pending request to a CA, the CA enrolls it and returns a certificate to the adaptive security appliance. After you have received the certificate, click Install and highlight the appropriate identity certificate to complete the operation.

To installing a pending identity certificate, perform the following steps:

What to Do Next

Configuring Code Signer Certificates

Code signing appends a digital signature to the actual executable code. This digital signature provides enough information to authenticate the signer, and ensure that the code has not been modified after being signed.

Code signer certificates are special certificates whose associated private keys are used to create digital signatures. The certificates used to sign code are obtained from a CA, in which the signed code reveals the certificate origin. You can import code signer certificates on the Code Signer pane, or choose Configuration > Remote Access VPN > Clientless SSL VPN Access > Advanced > Java Code Signer.

Showing Code Signer Certificate Details

To show detailed information about the selected identity certificate, click Show Details to display the Certificate Details dialog box, which includes the following three display-only tabs:

•The General tab displays the values for type, serial number, status, usage, public key type, CRL distribution point, the times within which the certificate is valid, and associated trustpoints. The values apply to both available and pending status.

•The Issued to tab displays the X.500 fields of the subject DN or certificate owner and their values. The values apply only to available status.

•The Issued by tab displays the X.500 fields of the entity granting the certificate. The values apply only to available status.

Deleting a Code Signer Certificate

To remove a code signer certificate configuration, select it, and then click Delete.

Note After you delete a certificate configuration, it cannot be restored. To recreate the deleted certificate, click Import to reenter all of the certificate configuration information.

Exporting a Code Signer Certificate

Step 2 Enter the name of the PKCS12 format file to use in exporting the certificate configuration.

Step 3 In the Certificate Format area, to use the public key cryptography standard, which can be base64 encoded or in hexadecimal format, click the PKCS12 format radio button. Otherwise, click the PEM format radio button.

Step 4 Click Browse to display the Export ID Certificate File dialog box to find the file to which you want to export the certificate configuration.

What to Do Next

Authenticating Using the Local CA

The local CA provides a secure, configurable in-house authority that resides on the adaptive security appliance for certificate authentication to use with browser-based and client-based SSL VPN connections.

Users enroll by logging in to a specified website. The local CA integrates basic certificate authority operations on the adaptive security appliance, deploys certificates, and provides secure revocation checking of issued certificates.

Configuring the Local CA Server

To configure a local CA server on the adaptive security appliance, perform the following steps:

Step 1 In the CA Server pane, to activate the local CA server, click the Enable radio button. The default is disabled. After you enable the local CA server, the adaptive security appliance generates the local CA server certificate, key pair, and necessary database files, and then archives the local CA server certificate and key pair in a PKCS12 file.

Note Be sure to review all optional settings carefully before you enable the configured local CA. After you enable it, the certificate issuer name and key size server values cannot be changed.

Step 2 When you enable the local CA for the first time, you must provide an alphanumeric Enable passphrase, which must have a minimum of seven, alphanumeric characters. The passphrase protects the local CA certificate and the local CA certificate key pair archived in storage, and secures the local CA server from unauthorized or accidental shutdown. The passphrase is required to unlock the PKCS12 archive if the local CA certificate or key pair is lost and must be restored.

Note The Enable passphrase is required to enable the local CA server. Be sure to keep a record of the Enable passphrase in a safe location.

Step 3 Click Apply to save the local CA certificate and key pair, so the configuration is not lost if you reboot the adaptive security appliance.

Step 4 To change or reconfigure the local CA after the local CA has been configured for the first time, you must shut down the local CA server on the adaptive security appliance by clicking the Disable radio button. In this state, the configuration and all associated files remain in storage and enrollment is disabled.

After the configured local CA has been enabled, the following two settings are display-only:

•The Issuer Name field, which lists the issuer subject name and domain name, and is formed using the usernameand the subject-name-default DN setting as cn=FQDN. The local CA server is the entity that grants the certificate. The default certificate name is provided in the format, cn=hostname.domainname.

•The CA Server Key Size setting, which is used for the server certificate generated for the local CA server. Key sizes can be 512, 768, 1024, or 2048 bits per key. The default is 1024 bits per key.

Step 5 From the drop-down list, choose the client key size of the key pair to be generated for each user certificate issued by the local CA server. Key sizes can be 512, 768, 1024, or 2048 bits per key. The default is 1024 bits per key.

Step 6 Enter the CA certificate lifetime value, which specifies the number of days that the CA server certificate is valid. The default is 3650 days (10 years). Make sure that you limit the validity period of the certificate to less than the recommended end date of 03:14:08 UTC, January 19, 2038.

The local CA server automatically generates a replacement CA certificate 30 days before expiration, which enables the replacement certificate to be exported and imported onto any other devices for local CA certificate validation of user certificates that have been issued by the local CA after they have expired.

To notify users of the upcoming expiration, the following syslog message appears in the Latest ASDM Syslog Messages pane:

%ASA-1-717049: Local CA Server certificate is due to expire in days days and a replacement
certificate is available for export.

Note When notified of this automatic rollover, the administrator must take action to make sure that the new local CA certificate is imported to all necessary devices before it expires.

Step 7 Enter the client certificate lifetime value, which specifies the number of days that a user certificate issued by the CA server is valid. The default is 365 days (one year). Make sure that you limit the validity period of the certificate to less than the recommended end date of 03:14:08 UTC, January 19, 2038.

In the SMTP Server & Email Settings area, you set up e-mail access for the local CA server by specifying the following settings:

a. Enter the SMTP mail server name or IP address. Alternatively, click the ellipses (...) to display the Browse Server Name/IP Address dialog box, where you can choose the server name or IP address. Click OK when you are done to close the Browse Server Name/IP Address dialog box.

b. Enter the from address, from which to send e-mail messages to local CA users, inadminname@host.comformat. Automatic e-mail messages carry one-time passwords to newly enrolled users and issue e-mail messages when certificates need to be renewed or updated.

c. Enter the subject, which specifies the subject line in all messages that are sent to users by the local CA server. If you do not specify a subject, the default is "Certificate Enrollment Invitation."

Step 9 Enter the CRL distribution point, which is the CRL location on the adaptive security appliance. The default location is http://hostname.domain/+CSCOCA+/asa_ca.crl.

Step 10 To make the CRL available for HTTP download on a given interface and port, choose a publish-CRL interface from the drop-down list. Then enter the port number, which can be any port number from 1-65535. The default port number is TCP port 80.

Note You cannot rename the CRL; it always has the name, LOCAL-CA-SERVER.crl.

For example, enter the URL, http://10.10.10.100/user8/my_crl_file.In this case, only the interface with the specified IP address works and when the request comes in, the adaptive security appliance matches the path, /user8/my_crl_file to the configured URL. When the path matches, the adaptive security appliance returns the stored CRL file.

Step 11 Enter the CRL lifetime in hours that the CRL is valid. The default for the CA certificate is six hours.

The local CA updates and reissues the CRL each time that a user certificate is revoked or unrevoked, but if no revocation changes occur, the CRL is reissued once every CRL lifetime. You can force an immediate CRL update and regeneration by clicking Request CRL in the CA Certificates pane.

Step 12 Enter the database storage location to specify a storage area for the local CA configuration and data files. The adaptive security appliance accesses and implements user information, issued certificates, and revocation lists using a local CA database. Alternatively, to specify an external file, enter the path name to the external file or click Browse to display the Database Storage Location dialog box.

Step 13 Choose the storage location from the list of folders that appears, and click OK.

Note Flash memory can store a database with 3500 users or less; a database of more than 3500 users requires external storage.

Step 14 Enter a default subject (DN string) to append to a username on issued certificates. The permitted DN attributes are provided in the following list:

•CN (Common Name)

•SN (Surname)

•O (Organization Name)

•L (Locality)

•C (Country)

•OU (Organization Unit)

•EA (E-mail Address)

•ST (State/Province)

•T (Title)

Step 15 Enter the number of hours for which an enrolled user can retrieve a PKCS12 enrollment file to enroll and retrieve a user certificate. The enrollment period is independent of the OTP expiration period. The default is 24 hours.

Note Certificate enrollment for the local CA is supported only for clientless SSL VPN connections. For this type of connection, communications between the client and the adaptive security appliance is through a web browser that uses standard HTML.

Step 16 Enter the length of time that a one-time password e-mailed to an enrolling user is valid. The default is 72 hours.

Step 17 Enter the number of days before expiration reminders are e-mailed to users. The default is 14 days.

Step 18 Click Apply to save the new or modified CA certificate configuration. Alternatively, click Reset to remove any changes and return to the original settings.

Deleting the Local CA Server

To remove the local CA server from the adaptive security appliance, perform the following steps:

Step 5 Choose one or more DN attributes that you want to change from the drop-down list, enter a value, and then click Add or Delete. Available X.500 attributes for the Certificate Subject DN are the following:

•Common Name (CN)

•Department (OU)

•Company Name (O)

•Country (C)

•State/Province (ST)

•Location (L)

•E-mail Address (EA)

Step 6 Click OK when you are done to close the Certificate Subject DN dialog box.

•If the user certificate lifetime period runs out, to remove user access, click Revoke. The local CA also marks the certificate as revoked in the certificate database, automatically updates the information, and reissues the CRL.

•To restore access, select a revoked certificate and click Unrevoke. The local CA also marks the certificate as unrevoked in the certificate database, automatically updates the certificate information, and reissues an updated CRL.

What to Do Next

Monitoring CRLs

Step 2 In the CRL area, choose the CA certificate name from the drop-down list.

Step 3 To display CRL details, click View CRL. For example:

CRL Issuer Name:

cn=asa4.cisco.com

LastUpdate: 09:58:34 UTC Nov 11 2009

NextUpdate: 15:58:34 UTC Nov 11 2009

Cached Until: 15:58:34 UTC Nov 11 2009

Retrieved from CRL Distribution Point:

** CDP Not Published - Retrieved via SCEP

Size (bytes): 224

Associated Trustpoints: LOCAL-CA-SERVER

Step 4 When you are done, click Clear CRL to remove the CRL details and choose another CA certificate to view.

Feature History for Certificate Management

Table 36-1 lists each feature change and the platform release in which it was implemented. ASDM is backwards-compatible with multiple platform releases, so the specific ASDM release in which support was added is not listed.

Table 36-1 Feature History for Certificate Management

Feature Name

Platform Releases

Feature Information

Certificate Management

7.0(1)

Digital certificates (including CA certificates, identity certificates, and code signer certificates) provide digital identification for authentication. A digital certificate includes information that identifies a device or user, such as the name, serial number, company, department, or IP address. CAs are trusted authorities that "sign" certificates to verify their authenticity, thereby guaranteeing the identity of the device or user. CAs issue digital certificates in the context of a PKI, which uses public-key or private-key encryption to ensure security.

The following paths were introduced, based on the type of VPN connection being used: